Straw Biomass Pretreatment And Cellulase-producing Fermentation Coupling Ethanol Biosynthesis

Cellulose biomass is the most abundant renewable resources in nature and it has great potential to safeguard global energy security in the future. Cellulase is a kind of complex enzyme, that can hydrolyze cellulose into simple sugars, which is mainly composed of cellobiohydrolase, endoglucanase and β-glucosidase. Cellulase was widely used in textile industry, feed, food industry, and bioenergy process. In this dissertation, the effects of pretreatment on the cellulase-producing fermentation in solid state and the components of straw substrate pretreated were investigated. The conditions of cellulase-producing fermentation and cellulose saccharification simultaneously were optimized. The condition of ethanol biosynthesis coupling cellulose saccharification was also investigated. The main contents and results showed as followings:(1) Straw powder was pretreated by dilute acid, dilute alkali, high temperature with dilute alkali, sulfite pretreatment overcome recalcitrance of lignocellulose(SPORL), sulfite pretreatment with dilute acid(SPORL with dilute acid). The effects of pretreatment method on the cellulases production by solid state fermentation and the yield of fermentable sugars hydrolyzed were investigated.The components of pretreated straw substrate, and the sugars in pre-hydrolyzed liquid were also detemined. The results showed that the straw substrate pretreated by dilute acid with SPORL for cellulase-producing fermentation, the carboxymethyl cellulase(CMCase)and β-glucosidase(β-G)activities produced reached 21511.22U/g and51508.41U/g, respectively, the hydrolysis rates of cellulose was 84.99%. The dilute acid method was better for removal of hemicellulose, and the hemicellulose content of pretreated straw substrate decreased from 20.77% to 7.34%. The high temperature with dilute alkali has a significant effect on klason lignin removal. The klason lignin content of pretreated straw substrate decreased from 12.47% to 7.58%. Further cellulose enzymolysis experiment showed that the hydrolysis rate of untreated straw powder was only 17.82%, while that of the high temperature with dilute alkali pretreatment could reach 91.66%. In the enzymolysis liquid of straw substratepretreated by dilute acid or SPORL with dilute acid, the content of pentosan was relative low, only 7.38% and 6.92%, respectively.(2) The conditions of cellulase-producing fermentation has been optimized.Single factor and response surface methodology were employed to optimize the conditions of cellulase-producing fermentation, in which the CMCase activity was used as index. The suitable conditions of cellulase-producing fermentation showed as following: the yeast Saccharomyces cerevisiae SP5 was inoculated while the White rot fungi NS75 and Aspergillus Niger NS83 had cultured for 2 days in the solid culture media, feed-water ratio 1: 2(w/v), inoculation volume ratio of White-rot fungi NS75, Aspergillus niger NS83 and Saccharomyces cerevisiae SP5 1: 2:1.5(v/v/v), the CMCase produced reached 24209.4 U/g.(3) The changes of content of reducing sugar were tracked and detected. The effect of inoculated yeast on cellulase-producing fermentation was investigated. The results showed that the reducing sugar content in the fermentation medium decreased significantly, and maintained in low concentration after the yeast inoculated, and the cellulase produced was significantly more than that of without yeast inoculated. The reducing sugar has the feed back inhibition to the production of cellulase. Inoculated the yeast could consume the reducing sugar of fermentation system and extend the phase of enzyme production.(4) Straw biomass pretreatment and cellulase-producing fermentation coupling ethanol biosynthesis. When straw powder pretreated by sulfite pretreatment with0.5% dilute acid, the production efficiency of cellulase, cellulose hydrolysis and ethanol biosynthesis were perfect. When hydrolysis sugar concentration 50.15 g/L,pH 6.0, inoculation volume 8%, 32.5℃ fermentation 72 h, the ethanol concentration produced in the fermentation liquid reached 28.3 g/L.